Fillers and filler compositions are well known and widely used in paper making applications in order to reduce paper costs by replacing relatively expensive virgin cellulosic fibers by less expensive filler. Fillers also make it possible to improve certain paper properties such as, for example, surface smoothness, printability and optical properties like opacity and brightness. However, other paper properties may be adversely affected. For instance, filled paper usually exhibit lower strength properties compared to unfilled paper.
Filled paper is conventionally produced to meet a certain specification, e.g. tensile strength, tensile stiffness, Scott Bond, thickness, grammage, filler content, etc. When producing paper at a specified grammage, it has been observed that the paper thickness is reduced when the filler content is increased. Paper thickness out of specification may give rise to handling problems, for example in high-speed conversion and end-use operations.
A thickness reduction in the filled paper may be compensated by increasing the grammage, i.e. increasing the amount of cellulosic fibre and filler used in the process in the proportions according to the specification, meaning that the overall production costs will increase. A thickness reduction may also be compensated by replacing part of the regular sulphate/sulphite pulps by bulky pulps, e.g. BCTMP (bulky CTMP). However, there may be drawbacks and problems associated with the use of such pulps.
Accordingly, there is still a need for processes for the production of filled paper and board with improved thickness control, improved runnability in high-speed conversion and end-use operations, and improved or maintained strength properties. There is also a need for processes for producing filler compositions which impart the above properties and advantages to paper and board.